Silencer and Open-Structured Catalyser

ABSTRACT

The invention relates to an apparatus arranged within a gas flow system, said apparatus comprising a matrix structure composed of a plurality of solid elements being arranged singularly or in groups. The gas flow system comprises a plurality of voids in-between said solid elements. The matrix structure allows gas to propagate within said matrix structure, and the matrix structure is intended for changing the chemical composition of said gas having passed through the system. The change in chemical composition either takes place during one or more of the following processes: one or more chemical reactions promoted by catalytic coating of said elements, promoted mixing of different phases of components of said gas, or phase change by fluid drops evaporating or solid particles changing phase, or takes place by the matrix structure having a degree of open structure of said matrix structure for allowing the gas to flow along an overall flow pattern being a not-truly 3-dimensional flow pattern.

FIELD OF THE INVENTION

The invention relates to apparatuses intended for performing one or more chemical reactions within a gas, such as catalytic conversion and/or retaining particles suspended in the gas, as well as to silencers incorporating such functions additional to that of suppressing noise.

BACKGROUND OF THE INVENTION

While previously beds of pellets or the like covered by a catalytic coating were commonly used to perform catalytic reactions in a gas flowing through such a bed, honeycombs with parallel channels have nowadays instead become the most common carrier for catalytic materials, gas flowing inside the channels in close contact with catalytic layers covering the inner surfaces of the channels. When the prime object is to carry out catalytic functions, not retaining suspended particles, the channels will usually be open at both ends, and the walls will be impervious to gas. A catalytic honeycomb can be manufactured in a number of ways, one very common method being to rely on wrapping a corrugated foil into a helical configuration that will appear substantially as a cylinder. Other well-known methods can be used to manufacture honeycombs of other shapes, such as for instance a hexagonal cross-sectional shape that is particularly useful in big catalysers composed of a multitude of smaller honeycomb modules.

Small honeycombs are generally easier and cheaper to manufacture than big honeycombs, especially with some manufacturing processes; an example in point is when a honeycomb is manufactured by extruding a ceramic material. A range of standardised sizes of cylindrical honeycombs, up to certain maximum diameters, have evolved in industrial practice. To use these standard products instead of tailor-made honeycombs of large size, odd shape, etc. in many situations will represent a number of practical advantages. Thus, for instance, a designer of a truck silencer capable of performing exhaust gas after-treatment will often prefer to arrange an array of relatively small, cylindrical honeycombs to be through-flowed in parallel. One advantage is that the price may be kept low, due to mass-fabrication. A second advantage is that there may be several alternative or supplementing sub-suppliers to choose from. A third advantage is that the reliable functionality of standard-manufactured honeycombs is based on a long record of operation, minimising the risk of many sorts of problems, including mechanical integrity of the units.

Filters for retaining particles suspended in a gas, for instance particles of un-burnt fuel, mainly consisting of carbon, suspended in the exhaust from a combustion engine, are also usually designed as honeycombs. The most common type, known as the wall-flow filter, is often manufactured as a ceramic monolith, for instance from cordierite or SiC. Roughly half of the channels will be closed at one end, and the other channels will be closed at the other end. The walls between the channels are made with pores, so that gas can penetrate them, while the pores will retain particles. Since no channel is open at both ends, gas will be forced to pass from a channel being open at the end of incoming gas onto a neighbour channel whose front end is closed, but whose rear end is open.

Combustible particles retained in the pores of a wall-flow filter should be oxidised so that they can escape in gas form, mainly as carbon-dioxide, and build-up of excessive pressure drop of the filter may be prevented or at least delayed. For such purpose, catalytic materials which promote oxidation may be added to a wall-flow filter.

A variety of honeycombs or the like that can perform both catalysis and retaining particles can be manufactured from metal foils that are wrapped up and shaped in various ways so as to guide the gas through the unit in flow patterns that may take many forms, depending on details of the structure. The foils will often be provided with perforations. One particular form of such a honeycomb-like structure may be characterised as a ‘half-open’ filter. By virtue of perforations and local deformations of the foil material adjacent to the holes, gas will both flow in channel-like passages and be forced to flow through perforations in a tortuous way. Heat-resistant mats are arranged within the foiled structure in such a way that gas is forced to impinge upon these mats. Thereby, due to ballistic forces, gas-suspended particles will penetrate into the mats to be captured, while the gas bends and flows further, by-passing the mat.

Although in some cases gas flow may locally be transgressing a wall, a foil or the like, as has been exemplified by the wall-flow filter and the half-open filter, in the following, the term ‘honeycomb’ will refer to a structure that has been designed so that the overall flow pattern is characterised by a flow from an inflow side of the structure to an outflow side, along flow-lines extending inside substantially parallel passages or channels.

Although honeycombs have proven very useful in many respects, there are some problems associated with them:

A very common problem associated with honeycombs, when installed in silencers or in chemical industrial plants, is that there may be deviations from a uniform flow from the inflow side to the outflow side, i.e. there will be a flow maldistribution in the honeycomb or an assembly of parallel honeycomb modules.

Honeycombs are generally arranged inside casings. Pipes or passages leading gas to and from such a casing will normally have transverse dimensions that are significantly smaller than those of the honeycomb or the assembly of honeycomb modules. When space is ample, one can make the casing considerably longer than the length of the honeycomb(s), in which case differences in honeycomb and passage cross-sectional areas may represent no great problem. However, space is often restricted, so that upstream and downstream distances from the openings of the casing to the honeycomb(s) will be short. In particular, a short distance from the inlet passage opening to the honeycomb(s) often represents a problem, since incoming flow of a relatively big velocity will have a tendency to impinge upon the honeycomb(s), causing relatively more gas flow to pass through the part being impinged.

One important application in which this problem is especially difficult is silencers comprising honeycombs. For the purpose of reducing noise of low frequencies, silencers often comprise two or more chambers which are interconnected by passages that must be sufficiently long and of a sufficiently small cross-sectional area compared to the cross-sectional area of the chambers inside which honeycombs can conveniently be accommodated. The relatively small cross-sectional area of the passages will then tend to cause particularly strong impingement of the kind that has been explained above.

Another common source of honeycomb flow maldistribution is that a passage may lead gas to a casing in a direction that deviates strongly from that of the channels of the one or more honeycombs. Thus, for instance, flow may be led in from a perpendicular direction. A silencer feature that has been found very useful in later years, since it allows for a long passage to be accommodated inside a rather small casing, is to accommodate a helically extending passage to connect two chambers. Such a passage may for instance be arranged to extend around a cylindrical honeycomb. In such a case the flow situation in front of the honeycomb may be that of a tangential, radial inward flow all the way around the periphery of the honeycomb.

Catalyser honeycombs in which both ends of the channels are open are especially sensitive to flow maldistribution. Wall-flow filters are less sensitive for two main reasons; firstly, the generally higher flow resistance in wall-flow filters helps even out flow maldistribution. Secondly, if some channels are swept by a bigger flow than other channels, there will be a tendency for more particles to accumulate in those channels, which in turn will slow down flow in those channels. In other words, wall-flow filters may to some extent provide an automatic self-repair of a flow maldistribution. Still, flow maldistribution is not without problems even with wall-flow filters. Thus, a continuous oxidation of accumulated particles, known as regeneration, may suffer from flow maldistribution, which contributes to making the regeneration less continuous, and to making the filter operate with a higher time-mean pressure drop.

One known method of promoting a more equal flow distribution across the inlet to a honeycomb configuration contained within a chamber is to insert a perforated plate or a screen between the inlet and the honeycomb(s). The bigger the pressure drop across the plate or screen, the more effective the evening-out effect will be, but there is often a penalty associated with a big pressure drop. In the case of the exhaust system of an engine, there will be an adverse effect on engine performance. In the case of a catalyser of a chemical plant, more fan power will be required to force gas flow through the apparatus.

Another known method relies on the use of a diffuser arranged at the inlet to a casing containing a honeycomb arrangement. Diffusers are attractive in two main respects; due to a widening flow area in flow direction, diffusers will slow down the flow which in itself will promote a more even flow distribution. Secondly, across diffusers that operate as they should (i.e. not exhibiting flow separation within the diffuser) there will be negative pressure drop across the diffuser, known as pressure recovery. The main problem with diffusers is that they tend to require space that may not always be available to the extent one would prefer from a fluid-dynamic point of view.

A third method that is less common, but which has been suggested in prior art, is to shape honeycombs with an inlet surface that deviates from a plane surface. Thus, for instance, it has been suggested to use a conical or tapered inlet shape of honeycombs to adapt them to the shape of an inlet diffuser. Such solutions could deserve consideration in cases of an incoming flow being more or less in line with the overall flow direction inside the honeycomb. However, cost reasons may detract from this type of solution. Also, even theoretic, odd-shaped honeycombs cannot be adapted to all kinds of difficult flow situations.

A common characteristic of the problems described above is that honeycombs, although being very fit for a number of chemical/particle retaining processes as such, and although being available on the market at comparatively low price, pose difficulties from the point of view of attaining optimal flow conditions. This aspect is especially prominent when space is limited, and silencers, due to geometric requirements prompted by acoustic considerations, often present the designer with especially difficult flow problems when honeycombs are to be inserted and operate optimally in respect of pressure, specific inflow or outflow directions, etc.

Grid or netlike catalysers are known to possess a number of appealing characteristics; they can accommodate more catalytic surface per unit volume than honeycomb catalysers which have been utilised, either for the purpose of creating a more compact catalyser or with the scope of attaining a smaller pressure drop within a given volume. Also, such structures have been appreciated for their capacity to promote more intensive kinetic activity adjacent to the catalytic layer, and grid or netlike structures are known to respond more rapidly to temperature changes.

SUMMARY OF THE INVENTION

An important aspect of the invention relies on the insight that grid or netlike structures or other open structures, due to a facility of permitting flow to propagate through the structure much more freely in space than in a honeycomb, can solve many of the kinds of flow problems pointed out above.

In this summary we shall be referring to these open structures as ‘matrix structures’; in the claims a strict definition of this concept is given. It is true that classical beds of pellets or the like share a facility for allowing flow to propagate more freely in various directions. However, pellets or the like have to be contained in some way, which makes them less attractive than the kind of coherent matrix structures that are covered by the invention.

A further aspect of the invention is that such matrix structures may be adapted for supplementing honeycomb arrangements, to obtain combinations of better fluid-dynamic flow situation within the honeycombs, to accommodate more chemical treatment capacity, such as catalysis, within a given available space, and a number of additional attractive features that will be demonstrated below.

As an example of this approach, in some cases supplementing a honeycomb by a matrix structure may surprisingly lower the pressure drop across an apparatus.

Furthermore, as will also be illustrated, the invention provides the possibility to arrange matrix structures or elements of matrix structures inside honeycombs to boost their performance in various ways. In such arrangements matrix structures provide hitherto unnoticed possibilities of arranging different catalytic coatings very close to each other, which may be utilised for boosting chemical reactions.

Matrix structures may advantageously be accommodated inside silencers in a variety of ways, some of which have been mentioned already. A still further aspect of the invention, related to silencers, is that matrix structures have been found to have a capacity for affecting the acoustic and fluid-flow performance of passages connecting acoustic chambers in a favourable way.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art silencer with incorporated honeycombs.

FIGS. 2 a and b show a first embodiment of the invention.

FIG. 3 shows a second embodiment of the invention.

FIG. 4 shows a third embodiment of the invention.

FIG. 5 shows a fourth embodiment of the invention.

FIG. 6 shows a fifth embodiment of the invention.

FIG. 7. shows a prior art silencer with no chemical treatment elements.

FIG. 8 shows a sixth embodiment of the invention, being a modification of FIG. 7.

FIG. 9 shows a seventh embodiment of the invention.

FIG. 10 shows an eight embodiment of the invention.

FIGS. 11 a and b show a ninth embodiment of the invention.

FIGS. 12 a and b show a tenth embodiment of the invention.

FIGS. 13 a and b show an eleventh embodiment of the invention.

FIG. 14 shows a twelfth embodiment of the invention.

FIG. 15 shows a prior art honeycomb with a diffuser inlet to a chamber. Below FIG. 15 a graph is shown of pressure variations according to FIGS. 15 and 16.

FIG. 16 shows a thirteenth embodiment of the invention.

FIG. 17 shows a fourteenth embodiment of the invention.

FIG. 18 shows a fifteenth embodiment of the invention.

FIG. 19 shows a sixteenth embodiment of the invention.

FIG. 20 shows a seventeenth embodiment of the invention.

FIG. 21 shows an eighteenth embodiment of the invention.

FIG. 22 shows a nineteenth embodiment of the invention.

FIGS. 23 a-c show a twentieth embodiment of the invention.

FIG. 24 shows a twenty-first embodiment of the invention.

FIGS. 25 a and b show a twenty-second embodiment of the invention.

FIG. 26 shows a twenty-third embodiment of the invention.

FIG. 27 shows a prior art silencer with built-in SCR DENOX facility.

FIG. 28 shows a twenty-fourth embodiment of the invention, being a modification of FIG. 27.

FIGS. 29 a and b show a twenty-fifth embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an example of a prior art silencer design. A silencer is provided through an outer casing 1 composed of a cylindrical shell 2 and two end caps 3 and 4, both provided with openings 5 and 6 of sizes a1 and a2, respectively, for leading gas to and from the silencer via pipes 8 and 9, respectively. To achieve a certain amount of silencing, the circular cross-sectional area A of the cylinder is significantly bigger than the cross-sectional areas of the openings. This is achieved by designing the cylinder with a rather big diameter D. In the section of the silencer shown in the figure, three parallel catalysers 30, 31, and 32 are enclosed between two walls 12 and 13 which together divide the interior of the silencer into two separate, acoustic chambers 10 and 11. All gas is forced through the catalysers. The catalysers are composed of a multitude of parallel channels 33. In order to achieve an even flow into all channels of the catalyser, there must be a certain distance d from the inlet opening of first chamber 10 to the channels 33.

Within a given total catalyser volume, the amount of catalyst surface for the conversion of gas or particles is restricted by considerations on channel level, and the permissible total volume of catalysers is restricted, especially if a reasonably even distribution of gas among various channels is prescribed. In theory, channels could be made of a very small diameter, but this would produce an excessive amount of pressure drop across the catalysers.

FIGS. 2 a and b show a first embodiment of a silencer according to the invention. A silencer is shown with an outer, squared casing 1 and inlet and outlet openings 6 and 7 of cross-sectional sizes a1 and a3, respectively. An internal divisional wall 6 divides the interior of the silencer into two acoustical chambers 10 and 11. Both chambers are of a significant size and of a significant cross-sectional area A due to a significant transverse size D of the casing of the outer shell. Inside the silencer, flow is passed from the first to the second chamber via an internal passage 5 of a cross-sectional area a2 and of a length LP. The tail pipe 9 leading gas from the silencer to the atmosphere has a length LT.

Inside the first chamber 10 a matrix-structured catalyser 50 consisting of a succession of plane screens or grids or nets 52 is arranged to almost fill out the entire space of the first chamber. All surfaces of the screens are covered by a catalytic layer.

A face-view I-I of a screen is shown in FIG. 2 b. A plurality of elongated members 53 extend in the horizontal direction, and a plurality of elongated members 54 extend in the vertical direction. Thereby, voids 55 are created in-between the elongated members. The screens are fixed to the casing at some distances between screens, creating plane voids 56 extending between two screens. The distance d from the inlet opening to the first matrix structure 50 is much smaller than the transverse size D of the casing. The same could have applied if the casing had been a cylinder of diameter D. Due to the open structure of the succession of screens, allowing for easy flow in directions perpendicular to flow chamber inlet to chamber outlet (i.e. horizontally in FIG. 2 a), all (or at least almost all) parts of the matrix structure which is made up of the screens will be swept by gas, thus contributing to catalytic function.

Inside the second chamber 11 another type of grid or net 51 has been arranged. Here, additional elongated members 55 are arranged in the axial direction, as appears from FIG. 2 a. Due to these extra members, the amount of catalyser surface area per unit volume is bigger in structure 51 than in structure 50. Structure 51 may be characterised as a 3-dimensional lattice. It should be noted that in both chambers 10 and 11 no gas is allowed to by-pass the catalysers, since such by-pass would compromise the catalytic functions.

The first embodiment of the invention illustrates that silencers provide an excellent example of utilising facilities of control of fluid flow inside a catalyser, provided by matrix structures. The better the desired acoustic performance, the bigger should be the ratios between chamber cross-sectional areas A (=D×D in FIG. 2 b) and passage areas a1, a2. On the other hand, the smaller the size of especially a1 and a2, the more prominent will be the tendency for flow to impinge on catalysers arranged downstream. If the two matrix structure catalysers had been honeycomb catalysers, flow mal-distribution represented by more flow through the core areas of such honeycombs would have been very prominent. The matrix structures, easing flow distribution across the entire inflow area to a catalyser, are capable of alleviating this problematic tendency.

Due to the openness of both matrix structures, the two chambers 10 and 11 will acoustically function as chambers, not being effectively disrupted by the presence of the matrix structures. The three arrows pointing at a space upstream of matrix 51, matrix 51, and the space downstream of 51, respectively, by way of illustration underscores this aspect. The silencer shown will function as an acoustical low-pass filter according to a well-known formula: f=(c/(2π)sqrt((a2/LP)(1/V1+1/V2) where c is the speed of sound, V1 is the volume of chamber 10, and V2 is the volume of chamber 11. If f is low enough, noise reduction will become effective in a broad acoustical spectrum, including relatively low frequencies that are higher than f. f is sometimes termed ‘local natural’ or Helmholtz frequency of the filter.

The volumes of the solid parts of matrices 50 and 51 will surely detract from acoustically effective volumes V1 and V2, respectively, but this effect will not be severe due to the relative openness of the two structures. The main acoustic effect of the matrices will be beneficial, since the matrices will tend to dampen standing waves set up in the chambers, waves that will have pressure nodes inside the chambers and maximum pressure variation at the walls, as the person skilled in the art of acoustics will appreciate.

Honeycombs with channels that are open at both ends, arranged inside chambers may also be viewed as relatively open structures from an acoustical point of view. Thus, if a honeycomb catalyser of this sort substantially covers the entire cross-sectional area A of a chamber, it may be justified to regard the chamber as en entity in acoustical considerations, for instance when applying the above formula.

Wall-flow filters, in which gas is forced through pores at a relatively high flow resistance, and which provide such a great resistance to acoustic waves, may be dubious in this regard, that is, when they are arranged inside a chamber it may be dubious, at least in some circumstances, to regard such a chamber as an acoustic entity. Especially if a wall-flow filter is composed of a plurality of cylindrical modules, being through-flowed in parallel, a pronounced acoustical effect may be obtained from dividing a chamber into two parts. This is because there must be walls or the like between the modules in order to prevent gas flow from by-passing filter modules.

This discussion of to which extent various types of gas treatment equipment will alter acoustic function of chambers illuminates an acoustically attractive feature of matrix structures arranged inside silencers to add chemical treatment of gas to silencing functions: if a certain degree of chemical change is provided within a silencer, such as a certain percentage of reduction of a certain component of gas flowing through the apparatus, a matrix structure, such as the grids or nets shown in the first embodiment of the invention, provides a possibility of affecting acoustically effective volumes of chambers to a minimal degree. The effect on low-frequency performance may even be positive. Sound corresponding to natural frequencies f (according to the formula) often passes through a silencer at a relatively lower degree of damping. The lowest natural frequency may even be slightly amplified by the silencer. Inserting a matrix structure inside a chamber will introduce a small, but not always insignificant damping of such frequencies.

The acoustically favourable effects described may be attributed to two fundamental aspects of matrix structures of the kind covered by the present invention: firstly, the surface-to-volume ratio is relatively big, and secondly, these structures may be adapted to cause greater kinetic activity in the gas close to the surface. An example of this may be if the two matrix structures 50 and 51 are both composed from threads, as the person skilled in the art will appreciate. Both these aspects also account for favourable aspects of matrix structures from a chemical point of view, as has been pointed out already in the last paragraph explaining the background of the invention.

Screens according to structure 50 may be stamped out of sheets of metal, in which case the voids that appear in a face view as shown in FIG. 2 b may be square as they are shown to be, or they could be circular or of many alternative shapes. Furthermore, a screen may be manufactured from a plurality of threads or strips (for instance round or flat) that are woven together. Screens or perforated plates are manufactured in mass-production for many applications. Therefore, layered matrix structures, like the one illustrated by structure 50, may be manufactured very cheaply.

Both structures 50 and 51 are shown to be organised in regular patterns. Alternatively, they could organised in a partly or fully random manner. One example is a metallic ‘sponge’ composed of intermingling treads and having an envelope as a lump or a flat, round sheet. Typically, such truly 3-dimensional structures will have envelope dimensions that are significantly bigger that the biggest cross-sectional dimension of the individual threads, fibres, strips or the like from which the structure are made.

FIG. 3 shows a second embodiment of the invention. Here, as in the acoustic chamber 10 of FIG. 1, a succession of plane screens are arranged to provide a matrix structure 50, the grids being covered by a catalytic layer. A first screen 51 has been arranged at a distance from the next screen 52 which is than the individual distances between screens making up part 53 of the entire structure 50. Thereby, the first plane void 54 is thicker than the downstream plane voids 55. Thus, there is an inlet section of the catalyser where it is particularly easy for gas to flow in transverse directions. This tendency could have been even more pronounced if voids (55 in FIG. 2 a) between solid members of the screens had been made smaller, for example for the two first grids; that is, if these two nets were made with closer meshes. The associated higher flow resistance in the horizontal direction in FIG. 3 would help spread out the flow towards the casing right from the gas flow inlet to the matrix structure 50.

FIG. 4 shows a third embodiment of the invention. Here, a catalyser 50 is constituted by a plurality of parallel passages or channels 51, separated by walls 52 that are perforated by holes 53. These holes will help flow to proceed in transverse directions. It may be seen that the holes have been arranged adjacent to each other in the transverse direction, which further reduces resistance to flow in the transverse direction across several channels. Once again, compared to honeycombs with impervious walls, where part flows proceed inside parallel channels only, it becomes possible to insert more catalyser volume within an acoustical chamber, retaining a reasonably even sweeping of all catalytic surface parts inside the catalyser.

No cross-sectional view of this third embodiment is shown. The walls and the passages could be planes that extend from one side of the casing to the opposite side, in a direction perpendicular to the plane of FIG. 4. In this case flow distribution within the planes will be very efficient. Such an arrangement could be appropriate if the size of the casing in this direction was significantly bigger than the size in the vertical direction of FIG. 4. An alternative that could for instance be applied inside a square or cylindrical casing (as in the first embodiment of the invention), could be to have square channels, that is to supplement by perforated walls in directions parallel to the plane of FIG. 4. In the case of a cylindrical casing, the entire perforated structure could be made from wrapping up a perforated, long sheet of metal.

At the end of the description, embodiments of the invention will be shown that constitute other matrix structures. These matrix structures also have the feature of plane sheets extending in a longitudinal direction of flow from one side of a matrix structure to the opposite side.

FIG. 5 shows a fourth embodiment of the invention. Here, an inlet pipe 5 leads up to an inlet opening 6 of a chamber 10, the opening being of size a1 and covered by a matrix open-structured catalyser 50 that completely covers opening 6, so that all gas passes this catalyser, and the flow generally diverges from the opening to spread out into the acoustical chamber. Also, a second catalyser 51 is shown to cover the outlet opening 7 (of size a2) from the chamber, so that all gas passes this catalyser by a flow field by which flow generally converges towards this opening, gas leaving the chamber via pipe 9.

FIG. 6 shows a fifth embodiment of the invention. A perforated pipe 7 is arranged within a silencer casing 1, the perforated pipe constituting a continuation into the casing of inlet pipe 8 and a continuation backwards into the casing of outlet pipe 9. An internal division wall 12 blocks flow within the perforated pipe, so that flow from the inlet pipe is forced out into the chamber by passing perforations of pipe 7, and flow is forced back into the pipe via perforations of pipe 7 to the right of the division wall, to leave the chamber via outlet pipe 9. Around this perforated pipe a matrix structure catalyser 50 has been arranged, for instance by wrapping one or more plane structures made up of grids, to cover all perforations, so that all flow will pass this structure when passing through the chamber. Some paths FP1 of the flow will flow entirely inside the catalyser, while other flow paths FP2 will leave the catalyser to flow within the empty space surrounding the catalyser.

FIG. 7 shows a prior-art silencer without any internal catalysers, while FIG. 8 shows a somewhat modified counter-part silencer with an inserted catalyser, being a sixth embodiment of the invention. The casings and most parts of the two silencers are identical or almost identical. In FIG. 7, a division wall 4 divides the interior of the silencer into two silencer chambers 10 and 11. A helically shaped passage 8 of rectangular cross-section, connects the two chambers, extending on the inside of the cylindrical shell part 2. By using a helical passage, it is possible to insert a rather long passage inside the silencer, which promotes low-frequency attenuation of noise. The helically shaped passage 6 extends 360 degrees around the axis of the silencer, gradually increasing its cross-sectional area in the peripheral flow direction, which creates a pressure recovery effect, i.e. the helical passage is a diffuser. The diffuser shape appears from the passage outlet 7 being bigger than passage inlet 6. Both in chamber 10 and in chamber 11, sound-absorptive material 13 has been provided, inserted between the outer shell and inner, perforated and cylindrical shells 13.

In FIG. 8, representing a sixth embodiment of the invention, a matrix-structured catalyser 50 has been inserted into the helical passage. In theory, it would have been possible to insert a ceramic monolithic catalyser of the same over-all shape, but this would have been much more expensive and cumbersome to arrange. The matrix structure can be manufactured as a straight, elastic rod that, when inserted into the curved passage, will bend to follow the passage.

In the particular embodiment, the catalyser is an SCR catalyser for promoting conversion of NOx by chemical reactions between NOx and urea, injected via a nozzle 30 arranged inside the pipe 8 leading up to the silencer. In order to avoid condensation of vapour on the inside of the outer shell of the first chamber, the perforated inner cylinder inside the first chamber 10 has been replaced by a solid cylinder 15 and material 14 (for instance mineral wool) providing sound and heat insulation, but no sound absorption. In the second chamber, the sound-absorptive function has been retained by having sound-absorptive material 12 being contained between outer cylinder 2 and perforated, inner cylinder 13.

The matrix structure 50 may increase the pressure drop of flow passing the helical passage, even though, surprisingly to a person not being very skilled in the art of fluid dynamics, the opposite could in fact be the case, as will be explained below. In any case, there will be minor flow separation on the ‘backsides’ of elements of the matrix, as in the classical example of a rod that has been arranged in a flow field. This has the acoustic advantage of suppressing resonances in this passage, above all the resonance which correspond to a half-wave set up in the passage from inlet to outlet (extending helically, as the passage itself). Also, a pronounced effect of damping Helmholtz natural frequencies f may be obtained. This effect may be explained in the following way: in a first approximation, particularly apt in relation to lower sound frequencies, the system may acoustically be viewed as a mass fixed onto two springs, the mass being that of gas contained within the connecting passage and the two connected chambers representing flexible springs. When the silencer is subjected to a sound of a frequency corresponding to the natural frequency of this spring, there will be a resonance effect that may even seriously compromise the performance of the silencer. But when a matrix is inserted into the passage, the dynamic, frictional resistance caused by the matrix will dampen this resonance.

By selecting a very open structure of the matrix, any pressure drop increase that may be caused by the structure can be reduced to a minimum. Another possibility is to make the cross-sectional area of the helical passage somewhat bigger, optionally to be combined with increasing the length of the passage somewhat, to maintain an unchanged local Helmholtz natural frequency f.

When a matrix structure, as has been explained above, is inserted into a helically extending passage, the relative increase in flow resistance will be smaller than if the same matrix structure had been inserted into a straight passage. The reason for this is that in the empty, helical passage, secondary swirl, within a cross-section, will be set up to increase the flow resistance compared to a corresponding, straight passage. Inserting a matrix structure in the empty helical passage will dampen such swirl or even eliminate it. Since circular passages are more prone to such secondary swirl, the fluid flow performance of such passages (instead of the rectangular passages of FIGS. 7 and 8) may benefit even more from insertion of a matrix structure.

In some cases insertion of a matrix structure inside a diffuser, straight or curved, may improve the fluid-flow performance dramatically. This happens when the diffuser has been made with a too great increase of flow area in flow direction, such as when the length is given, and the ratio between inlet and outlet passage cross-sectional area is chosen to be too large. In that case, flow separation will occur, that is flow will not really widen, but will cling to one of sides, as is well-known from elementary fluid mechanics. A matrix structure inserted in such a diffuser can eliminate such flow separation, provided the degree of area increase is not too big. Thus, if a passage between two chambers is made up of one part of a constant cross-sectional area and a second part being a diffuser, an optimal solution may be to insert a matrix structure in the diffuser part only.

FIG. 9 shows a seventh embodiment of the invention. Here, a first matrix structure 50 is arranged around an inlet opening, similar to FIG. 5. A second matrix structure 51 has been inserted into a helical passage 5, not filling out the cross-section, but being limited to an outer zone, while there is an open, inner part 52 of the passage. By thus filling up only part of the passage, a smaller increase in flow resistance may be obtained than if the entire cross-section had been filled up by the matrix structure. Due to centrifugal forces acting upon gas elements, the risk of gas elements passing through the passage without coming into contact with a catalytic layer of the matrix can be minimised.

FIG. 10 shows an eighth embodiment of the invention. Here, a matrix structure 50 has been arranged inside a second chamber 11 which has been almost filled up with a matrix structure 50 being a catalyser. The overall flow directional change in this second chamber is 90 degrees, i.e. more than 45 degrees as specified in one of the claims. Also, from inlet pipe 8 to outlet pipe 9, the flow turns more than 45 degrees. A pipe system configuration where there is a major change of flow direction inside a chemically and/or acoustically active apparatus is very common.

FIGS. 11 a and b show a ninth embodiment of the invention. FIG. 11 a is a longitudinal section of the silencer having a cylindrical shell 2, while FIG. 11 b shows an out-folding of an inserted matrix structure catalyser 50 on a fictive cylinder 11 inside chamber 10, to illustrate flow vectors up and downstream of the catalyser.

Gas is led into a cylindrical casing 1 via inlet pipe 8, the direction of which coincides with a tangent of the fictive cylinder, i.e. gas is led into the casing in a tangential direction. Thereby, a rotational flow field is created upstream of the catalyser. If the catalyser had been of the type with elongated, parallel channels instead of the matrix structure, the fluid flow transition from rotation to co-axial flow inside the catalyser would have been rather abrupt, and the centrifugal force upstream of the catalyser would have created a tendency for more flow to pass through channels adjacent to the casing shell, compared to channels closer to the centre of the catalyser.

Now the catalyser is instead of such a structure that flow inside the catalyser in addition to co-axial flow will be both tangential and radial, which is advantageous. Due to radial flow inside the catalyser there is an evening-out effect, so that the upstream tendency to a flow concentration toward the outer parts of the catalyser is gradually being reduced so that a more even radial flow distribution will exist at the outlet from the matrix. The rotational flow is gradually being ‘braked’ inside the matrix, by which dynamic energy of the rotation is being utilised for promoting catalytic activity, due to more intense gas kinetic activity between the catalytic layers and the bulk gas flow between the matrix structure solid elements.

FIGS. 12 a and b show a tenth embodiment of the invention, once again applied to a casing with a cylindrical shell 2. A curved, helically extending passage 6 connects an outer, annual chamber 10 with an inner, circular-cylindrical cavity 12 being part of a second acoustic chamber also comprising a cavity 14. The cylindrical cavity contains a matrix-structured catalyser 50. A cylinder 4 forms both an outer casing of this cavity and the inner, cylindrical part of the helical passage. Due to a tangential inflow to the outer chamber via pipe 8, gas will, in addition to moving axially, rotate around the axis of the silencer. Gas leaves the passage along a helically extending outlet section 13 from which it passes on to a flat, cylindrical cavity 14, where flow turns 180 degrees in an inward, radial movement, the gas flow being guided by vanes 15. The vanes will tend to take away the rotational movement, so that flow towards the catalyser, in an axial direction to the left in FIG. 12 a, will be more or less aligned.

The general arrangement of this embodiment will appear akin to some designs known from prior art, related to other catalysers than a catalyser designed as a matrix structure according to the invention. However, the matrix-structured catalyser allows for both tangential and radial movement inside the catalyser, once again causing an evening-out of the flow inside the catalyser.

Instead of vanes forcing the flow to stop its rotational movement around the axis of the silencer, less fluid-flow forcing vane shape could have been adopted, or one might even leave out the vanes; in the case of a honeycomb replacing the matrix structure, just leaving out the vanes may simply be unacceptable because of resulting flow maldistribution within the honeycomb.

A cavity 14 may be contained within a cap part 15 of the silencer that by not shown details, related to the transitional zone 16 of the cylindrical casing, can be arranged so that the cap is de-mountable.

FIGS. 13 a and b, representing an eleventh embodiment of the invention, once again show an arrangement which in its overall arrangement resembles prior art, but with the important distinction of the incorporation of two matrix-structured catalysers 50 and 51 instead of using honeycombs with parallel flows inside aligned channels. The differing matrix-structured catalysers will improve the function of the apparatus, partly due to effects already described in relation to the two foregoing embodiments of the invention. In addition, the catalyser 51 will cause an evening-out of the inflow to the honeycomb 30, which will improve its performance, compared to a design without the pre-catalyser 51.

From inlet pipe 8 flow enters the outer, annular chamber 10 by a tangential motion, so that a rotational movement is set up inside this chamber, upstream of matrix structure 50, and flow is distributed across the inlet to this structure. Inside this catalytically covered matrix structure, the rotational flow is being ‘braked’ (similar to what has previously been described), causing an added catalytic activity. From this first catalyser flow moves axially to the right inside annular cavity 12 towards a chamber-interconnecting member 20. This member contains two spirally extending passages 13 and 14, separated by spirally extending walls 15. Hereby, there will be a gradual inward, radial movement of gas.

As in previously shown silencers with helically extending passages connecting two acoustical chambers, gas will have to move a longer distance, compared to a simple, rectilinear movement inwardly; this improves attenuation of low-frequency noise. The chamber-interconnecting member 18 is delimited to the left by a wall 16 and to the right by a wall 19, both being parts of an end cap unit, containing helical passages 13 and 14 and being de-mountable as a separate unit, due to not shown details that the person skilled in the art will readily be able to supplement. The first wall 16 is provided with two openings 17 and 18, as indicated in FIG. 13 b, by which flow enters the end cap, to continue into passages 13 and 14 via passage inlets 21 and 22, respectively.

In the centre part 23 of the chamber-interconnecting member 20, gas leaves the two passages 13 and 14 in a tangential direction, which sets up a flow field of rotation in this centre part. In the case of a conventional arrangement with a central honeycomb only, for flow to pass from this flow field to an axial inflow in the left direction, to parallel channels of it would be necessary to arrange for some distance to the catalyser. Still, evening-out of flow will not be perfect, so that there will be different flow rates in differing channels of the catalyser.

By contrast, in the inventive embodiment shown here flow evening-out takes place in a matrix structure 51, arranged upstream the conventional honeycomb catalyser 30, and there is no need for a cavity. Thus, more catalyser can be accommodated within a given total space, and the conventional catalyser 30 will perform more effectively as a catalyser.

Dividing the central catalyser into two parts, as is done in this embodiment, provides the engineer with more freedom when optimising the catalytic effect. For instance, the pre-catalyser 51 may serve the purpose of finalizing mixing of urea that in the case of SCR-catalysis may have been injected into the silencer upstream of, or directly into passages 13 and 14 (such injections are not shown in the figure but are easily imaginable). Another feature of the embodiment will be that the matrix structure, due to its big catalytic surface-to-solid-volume ratio will respond more quickly to changes in mass flow rates, a feature that will be particularly useful when the apparatus is applied to vehicles, where the engine sometimes changes its mode of operation vary quickly, due to speed changes of the vehicle. The case of upstart from a standstill condition with a cold exhaust system is a particular case in point in this respect.

Instead of a honeycomb whose channels are open at both ends, the honeycomb 30 could have been a wall-flow filter. In that case, both catalysers 50 and 51, or one of them, could perform oxidation of NO into NO₂ that will promote regeneration of the filter. Since two catalysers are present, they may alternatively be differently coated, so that one of them is especially suited for oxidising other elements than NO. There might even exist a situation where a silencer, incorporating a honeycomb resembling the design shown in FIGS. 13 a and b, could be modified just by adding one or more matrix structures to improve performance, for instance by filling out a cavity in front of a central honeycomb by inserting matrix structure 51. This example and further examples that will be given below, illustrate that the great flexibility of matrix structures may be adopted in almost any silencer design or any design of an apparatus designed for performing chemical processes, can be enhanced by addition of matrix structures. Thus, for instance, a silencer only performing silencing, can be provided with much more

FIG. 14 shows a twelfth embodiment of the invention. Here, only part of a silencer is shown. A generally curved passage 6, comprising both curved and almost straight parts, leads gas from a first chamber 10 to a second chamber 12, the two chambers being separated by wall 4, and optionally further parts not shown. Contour 31 around a monolith 30 is rounded in a way that can be attained by adopting winding metal foils a way that creates two focal points 32 and 33. This is just one example out of many of how metal foils can be adapted to accommodate various features of a honeycomb. A structure of the kind indicated, when heated will create relatively small thermal stresses, as described in prior art documents published by the company Emitec GmbH. In addition, the cross-sectional shape provides better ‘filling out’ of the near-rectangular shape of silencer casing 2, compared to a single, circular honeycomb, or to an assembly of two or more cylindrical honeycomb modules.

Gas is led into the passage via inlet 1 and out via outlet 3. In the centre of the honeycomb a hole 7 has been indicated. Such a hole may be utilised for accommodating an internal tailpipe, i.e. a pipe that will extend perpendicularly to the plane shown, providing a connection from a chamber to the external environment. In such an arrangement, parts of the geometry may connect such a tailpipe with second chamber 11.

The curved passage has been formed within the residual space between the outer shell geometry 2 and the outer geometry 31 of the honeycomb. As can be seen, the cross-sectional area of this passage varies; in the ‘corners’ 7 the area is greatest. Here, lumps of matrix structures 50 have been inserted. Due to the micro-separational flow surrounding threads of the matrices, there will be no major flow separation in the corners, a phenomenon that most probably would have occurred in the absence of the matrices. Such major separation would cause significant pressure drops within the curved passage. Apart from preventing major flow separation there are two benefits associated with ‘micro-separation’ associated with this phenomenon: firstly, when the matrices are supplied with catalytic coating, such as a coating promoting oxidation, micro-separation will enhance this oxidation, due to lively kinetic activity around solid elements of the matrices. Secondly, there will be damping which will have beneficial acoustic effects.

It should be added that the embodiment shown in FIG. 14 represents a relatively cheap silencer configuration, in that silencer parts (the shell and the outer contour of the honeycomb) already in place are utilised for creating a passage. That is, a curved passage with its capacity for lowering a natural acoustic frequency, compared to a shorter, straight passage, has been created with a minimum of extra elements of the silencer. The possibility of inserting matrix structures into passages where there would otherwise be harmful or even catastrophic flow separation may be described as a way of creating a ‘forgiving’ fluid-dynamic design tool.

As the person skilled in the art will appreciate, a number of the previously described configurations, illustrated by application to silencers, may also be adopted in arrangements where chemical reactions are in focus and there may be no specific intention of suppressing noise, even though dampening of noise or pulsations in flow may occur as a by-product process.

Now some further embodiments will shown, without specific reference to silencers. However, the person skilled in the art will appreciate that they can all be accommodated in silencers. FIG. 15 is an example of a prior-art configuration where the idea of inserting a diffuser for the purpose of reducing mal-distribution across a honeycomb has been adopted.

Gas enters a chamber 1 containing a cylindrical honeycomb 30 whose channels are open at both ends. The cross-sectional area A of the honeycomb is much bigger than the areas of not only the passages leading gas to and from the chamber, but also compared to the outlet area of the diffuser 7. As is often the case when space is narrow, the inlet to the honeycomb has been arranged at a relatively small distance from the inlet to the chamber. Thus, there will be a tendency for the incoming flow to impinge onto the central part of the honeycomb, so here relatively more flow will pass through the channels. In other words, the residence time of gas within the channels is greater in the more peripherally positioned channels than in the central one.

Such flow maldistribution may be disturbing in any case of a honeycomb catalyser performing one or more chemical processes. The actual effects of flow maldistribution will depend on the specific reactions. In some cases, where chemical reactions depend on the degree of turbulence inside channels, there will be less activity in channels with a slower flow. Other processes, Depending less upon the degree of turbulence than on time, will suffer from a too short residence time of gas in the core region, resulting in less chemical conversion inside channels.

Tailoring matrix structures to the various types of reactions taking place will prompt different strategies. In the former case one can shape a matrix or two matrices, up and downstream of a honeycomb so that there will be relatively more added catalytic material in series with channels where flow is slower. Conversely, if the reaction is more time-controlled, one may fit in relatively more added catalyser capacity in series with channels of less residence time.

FIG. 16 shows a thirteenth embodiment of the invention, representing a modification of the prior-art configuration shown in FIG. 15. A matrix structure 50 is made up of a series of catalytically coated screens 51, 52, and 53 that have been arranged inside the casing upstream of the honeycomb. The diffuser of FIG. 15 has been replaced by a more widening diffuser 7, composed of a conical first part 5 and a round-contour part 6. Due to a pressure drop across the matrix and a smaller flow resistance in transverse directions, compared to the axial direction, the flow mal-distribution within the honeycomb has been diminished. In a core region of the matrix (parts 55 of sheets 52 and 53), the grid resolutions are more dense than in outside the core region, 54. Thus, flow passing through the core region will meet relatively more catalytic surface within the matrix, a feature that in some cases of catalytic reaction can make full compensation for the residual, small degree of flow mal-distribution. Thus, the initial mal-distribution can be compensated for in more than one way. Due to the method of arranging more catalytic material in the core region, it is not necessary to aim at full evening out of flow mal-distribution. In some cases a major emphasis can be placed on the method of arranging catalytic material within the matrix structure in a purposeful way, reducing the penalty due to pressure drop across the matrix to a minimum.

Between FIGS. 15 and 16 a pressure-line diagram has been inserted. Here, the full curve shows the static pressure line throughout the arrangement for the prior art configuration of FIG. 15, while the dotted line refers to FIG. 16. In diffusers static pressure increases gradually, while in flow restrictions, such as the honeycomb and the matrix structure, pressure gradually decreases. At inlet to the outlet pipe 9, static pressure drops rapidly, due to an acceleration of the flow and an accompanying increase of dynamic pressure at the expense of static pressure.

As can be seen, the total pressure drop across the entire arrangement, as taken from the inlet point upstream the diffuser to a downstream point inside the pipe leading gas from the casing, in the inventive embodiment has not surprisingly become smaller.

This can be attributed to the more widening diffuser. The comparison is fair, since considering the length of the diffuser to be given, in the prior art configuration it would not be realistic to adopt the more widening diffuser, since then flow inside the diffuser would be subjected to major separation, that it, it would not be possible for the flow to adhere to both the upper and the lower diverging contours. Due to the flow resistance arranged immediately in front of the diffuser outlet in FIG. 16 and a concentration of flow resistance to the core region, such flow separation is prevented in the embodiment of the invention.

The more widening diffuser, with outwardly rounding contour immediately upstream the interface with the casing, represents a third feature of assisting more even flow distribution across the inlet surface to the honeycomb.

FIGS. 17 a and b shows a fourteenth embodiment of the invention. Here, gas flow is led tangentially into a casing 1 in which a honeycomb 30 has its axis oriented perpendicular to the axis of the inlet pipe 8. As in FIG. 16, a diffuser 7 and a subsequent matrix structure 51 has been interposed between the inlet pipe and the honeycomb. The diffuser has an upper contour line 5 that provides a tangent to the circular casing. The lower contour line 6 of the diffuser bends, so that at its end point its direction is also that of a tangent of the circular contour of the casing. The matrix structure is composed of an inner section of concentric rings 51, while in an upper region, facing incoming flow from the diffuser, two arcs of grids 52 have been provided. These arcs are short enough for allowing flow bending downwards along flow-line 53 and the contour the lower part of diffuser contour line 6, to flow more freely here than along flow-lines in the upper region of the diffuser. This will help prevent flow separation on diffuser contour 6.

FIG. 18 shows a fifteenth embodiment of the invention. Here, flow approaches a cylindrical honeycomb 30 inside a casing 1 from a pipe 8 from an angle of around 45 degrees within the plane of the figure. Again, a diffuser 5 and a matrix structure have been interposed. The matrix is composed of a succession of circular, plane grid sheets, while in the front of diffuser outlet vertical grids 52 provide extra flow resistance to help prevent major flow separation within the diffuser.

FIGS. 19 a and b show a sixteenth embodiment of the invention, being part of a silencer. Inside a cylindrical casing 1, a helically extending passage 5 is wound around a cylindrical honeycomb 30. Flow exits the passage by flowing into annular spacing 6. From this spacing flow enters a circular matrix structure 50 with built-in guide vanes 7, shaped in a more flow-friendly way than the simple, straight vanes adopted in FIG. 13 b. Compared to various similar arrangements there is relatively little ‘breaking’ effect of the matrix structure. Thus, extending the matrix outwardly to fill up the annular spacing outside adjacent to the cylindrical casing would have caused more breaking effect. So would omission of the vanes. Whether to aim at a great or a small amount of breaking effect within the matrix is appropriate, will depend on the specific catalytic process being employed. When the process of such a nature that a lively kinetic actively is a great benefit to the process, a configuration with much breaking effect may be preferable. On the other hand, if little is gained in terms of added chemical conversion by a greater kinetic activity one may prefer to minimise the breaking effect and rather design for a low pressure drop across the apparatus. The embodiment shown in FIGS. 19 a and b exemplifies this latter situation.

FIG. 20 shows seventeenth embodiment of the invention, being a detail of a matrix structure. There are two groups of grids: A first group 31 with relatively sparse meshes and thin threads, and a second group 32 with relatively tight meshes and thicker threads. The first group can be regarded as spacers, ensuring that most parts of the relatively big surfaces of grids belonging to the second group are in contact with gas almost everywhere. Still, the relatively sparse meshes also represent surfaces that can be utilised for providing additional catalytic capacity. The two meshes could simply be provided with the same catalytic coating. Another possibility could be to use different coatings. Some chemical conversion processes will benefit from such a combination, both because different processes promoted by different catalysers can run concurrently, and because one part process will require more catalytic surface than another part process.

FIG. 21 shows an eighteenth embodiment of the invention. Here, coarser grids 31 with bigger threads are combined with more fine-graded grids 32 with thinner threads. The coarser grids can help provide mechanical stiffness within the structure.

FIG. 22 shows a nineteenth embodiment of the invention, being an arrangement where a grid 50 has been combined with a honeycomb 30, having open channels at both ends, in such a way that there are parts of the grid 51, 52 both up and downstream of the honeycomb, the two parts being interconnected by threads 53 extending all the way through channels 31 of the honeycomb. As can be seen, at channel in and outlets, threads 54, 55 have been dented in such a way that these dents will support positioning of the thread in the middle of the channel. The thickness of the thread is significantly smaller than that of the channel width. By virtue of such dimensioning, the thread inside the channel will only marginally increase the pressure drop along the channel.

An arrangement of this kind opens up for many possibilities of tailoring a catalyser in various ways. Three examples will be given here in broad terms: First, the thin thread within the channel represents are small thermal mass per catalyser surface unit, which helps speed up the thermal response of the assembly. Second, two different catalytic coatings can be arranged very close to each other by simple manufacture. Third, by deliberately arranging the thread very close to the wall of a channel or even to touch it, one might ‘short-cut’ chemical reactions that would otherwise be performed in separate steps if gas were to meet first one type of catalytic coating outside in one catalyser and then another type in a downstream catalyser. In other words, arranging a thread inside a channel opens up for concurrent chemical reactions without resorting to complicated procedures of arranging varying catalyser coatings onto inner surface walls of a honeycomb, such as for instance a checkered pattern of alternating coating that would be rather costly to manufacture.

FIGS. 23 a-c show twentieth embodiment of the invention. Here, a grid 51, being an example of a matrix structure, has been arranged right at the inlet to a honeycomb catalyser 30. As can be seen from FIG. 23 b, grid 51 is more fine-graded than the cells forming channel walls 32 (FIG. 23 c). At the rear ends of the channels, elements 53 serve to position the rear ends of threads 50 inside the channels. As an example, element 53 is shown to be a thread-like cross 54. The person skilled in the art will appreciate than many types of positioning elements, allowing gas flow to pass through such elements and providing various types of positioning, can be envisaged.

FIG. 24 shows a twenty-first embodiment of the invention being a wall-flow filter 30 inside whose channels 31 thin threads 50 carrying a catalytical coating have been arranged. Each thread has its one end 32 fixed within the closed end of the channel. In manufacture, this can be done concurrently with channel closing. The threads are being shown to be left otherwise free inside the channels, but could have been fixed in various ways as shown above.

The catalytic material of the threads can so that they will cause oxidation, such as oxidation of NO to NO₂. The walls of the filter may be catalytically coated, or that may not be considered necessary for a sufficient oxidation of accumulated particles and other combustible elements by NO₂ and O₂ to become effective. One might even envisage that singular oxygen molecules O in such an arrangement can be created, and will combine directly with combustible matter before combining with NO. If this is possible, a very efficient way of combusting accumulated particles of a wall-filter will have been achieved. This provides an example of what has above, as related to a honeycomb with both ends open, been designated ‘shortcut’ of chemical reactions or concurrent reactions.

FIGS. 25 a and b show a twenty-second embodiment of the invention, where thin threads 50 shaped as helical springs have been arranged inside channels of a wall-flow filter, to be held fixed by way of friction within the channels. As is commonly known, if a spring is subjected to torsion in one direction of turning, its outer diameter will shrink. By taking advantage of this, a simple tool for arranging the spring inside a channel can be designed. The tool can be composed of a cylindrical rod whose diameter is slightly smaller than the inner diameter of the spring when it is initially left free, arranged around this rod. A wrench can then apply torsion onto the spring whose diameter will shrink until it comes into contact with the rod. Then the assembly of the spring and the rod can be pushed into the channel. When the spring has been positioned axially, the torsion is released, the spring will expand onto the walls, and the rod can be taken out. If the channels 32 are not circular, but for instance squared as shown in FIG. 25 b, the total surface of contact areas 33 between the spring and the channels will only make up a minor part of the total surface of the threads, which may be preferable in some cases. On the other hand, if the channels are round, the contact area will be much bigger, and all parts of a thread will be either in contact with the walls or close to them, which will help promote the kind of concurrent chemical reactions referred to above.

Naturally, when elongated members or threads extend into channels of a honeycomb, they will increase the flow velocity inside the channels, which in some cases can be beneficial in that it may promote kinetic activity and thereby chemical processes. On the other hand, the higher flow velocity will increase the pressure drop. Especially in the case of a catalyser honeycomb with both ends open, where the pressure drop across the honeycomb without any insertions can be low, this may be a draw-back. Therefore, when threads are inserted in a honeycomb that has already been designed for a specific purpose, at will be advisable to use elongated members or threads that occupy only a certain part of the cross-sectional area of a channel, such as at the most 50%, or more preferably at the most 25% or even more preferably at the most 10% of the cross-sectional area.

On the other hand, when a honeycomb catalyser is being designed with the concept of inserting elongated members or threads, more channels or channels with somewhat bigger cross-sectional areas can be selected, to make a compensation that will lower the pressure drop across the combined arrangement.

The concept of having threads penetrating into a honeycomb offers a possibility of selecting shapes of individual threads, such as thickness or length of threads individually for the various channels, for the purpose of compensating for any mal-distribution of flow across the channels of the honeycomb.

The concept of arranging threads inside channels of a honeycomb represents a very space-economical way of enhancing the performance of a honeycomb, whether it be a catalyser with open ends and impervious walls, a wall-flow filter or any other kind of honeycomb.

The person skilled in the art will appreciate that this idea can be extended to having threads or any shape of elongated members being arranged into any kind of honeycomb structure, even kinds not described above. The company Emitec GmbH has invented a great many honeycomb designs created by shaping metal foils in many ways. One group of such honeycombs are represented by the previously described filter with mats inserted for capturing particles. But in a many disclosures by patent applications this company and other companies have shown that metal foil honeycombs can be adapted in numerous ways, to tailor the flow pattern through the honeycomb according to many design requirements. In almost any such design insertion of threads or elongated members can boost performance, for instance by the virtue of a thread responding more rapidly to changes in gas temperature, due to the small thermal mass of a thread, compared to a metal foil.

A last embodiment that is once again a silencer will be explained below. This embodiment represents a modification of a prior art silencer that will first be explained, as a background.

FIG. 26 shows a twenty-third embodiment of the invention, where different threads 50 are of differing lengths 51 and 52. The threads are fixed inside the channels by dents 54 and 55, like the arrangement shown in FIG. 22. In addition to accommodating catalytic capacity inside channels to enhance catalytic performance, this type of configuration offers a possibility of tailoring the flow resistance of individual channels without changing the honeycomb that can be manufactured as a standard product. Sometimes it will possible to compensate for a smaller flow area inside honeycomb channels, caused by inserted threads, by increasing the number of parallel channels, so that there will be no increase of pressure drop across the honeycomb.

FIGS. 27 a-c show a prior-art silencer with inserted catalysers adapted for performing SCR DENOX onto an exhaust gas being silenced as well. The apparatus outwardly appears with a box-like shell 1. Exhaust gas from a combustion engine is led to the apparatus via pipe 2 and from the apparatus via outlet pipe 3. Quite a distance upstream of the apparatus a nozzle 30 has been arranged for adding urea to the gas. The pipe leading up to the apparatus will allow for urea to mix with gas and evaporate before entering the apparatus. The interior has three cavities 4, 5, and 6, being created by flat division walls 7 and 8. In line with previous considerations, cavities 5 and 6 can be regarded as constituting a single acoustic chamber. An internal pipe 9 leads gas from the first cavity 4 through cavity 5 into cavity 6, the pipe providing full separation between internal pipe flow and chamber 5 by having impervious walls where the pipe leads gas through chamber 5. Pipe 9 inside chamber 4 has a perforated inlet section 10 with holes 11 through which gas enters the pipe from chamber 5.

In total 6 honeycomb catalyser modules 31 to 36 are have been arranged to be through-flowed in parallel. They are held fixed onto division wall 8 that has been provided with in total seven holes 37 of diameters adapted for arranging the modules as well as pipe 9. Each catalyser module consists of a central honeycomb 38, a cylindrical metal casing 39, and an intermediate heat-resistant and flexible layer 40. Further, each catalyser module has been divided into a first and major part 41, where catalyser channels have been supplied with a catalyst adapted so as to perform DENOX. A rear and small part, 42 has been provided with a different catalyst adapted for reducing ‘slip’, that urea rest products that have not been consumed in the upstream DENOX process.

Cavity 6 is a combined distribution and flow reversal cavity. Here, gas from pipe 9 is being divided into six part flows through each catalyser module, at the same time reversing its flow direction, as seen in planes B-B and C-C of FIGS. 27 b and c. Inside cavity 6, each module has been provided with a cylindrical cap 43 with a cylindrical pipe part 44 that has perforations 45 all way round, so that gas passing through these holes into the interior of cap 43 will come from all around the periphery of the module. When gas part flows have passed through the catalyser modules they will merge in cavity 5 before leaving the apparatus via outlet pipe 3. It is essential that openings 42 are not too big and too many, in order that there will be enough pressure drop across the holes to ensure that the 6 part gas flows are approximately equal, and the inflows to the interior parts of caps 41 are distributed approximately equally all around the peripheries, so that inflow distributions to the channels of each module will not be skewed.

An arrangement of the kind shown is quite common in prior art. Of course, more catalytic surface could have been built into the apparatus by selecting a rectangular honeycomb instead of the six cylindrical modules, but some honeycomb technologies at present simply do not permit designing for such a large and odd-shaped a honeycomb at a reasonable price, and even with those honeycomb technologies that offer better possibilities of adapting the shape of a honeycomb, there will be a significant price reduction associated with using smaller cylindrical modules that have been standardised in catalyser industry; the company designing the apparatus will have a whole range of alternative catalyser suppliers to select from when choosing the specific sub-supplier and catalyser technology.

FIGS. 28 a-c show a twenty-fourth embodiment of the invention, representing a modification of the prior-art apparatus of FIGS. 26 a-c. The following changes have been made:

The urea nozzle 30 has been arranged to supply urea inside the apparatus instead of into inlet pipe 2. An inlet diffuser 21 has been added to inlet pipe 2, to provide a transition form this pipe to the inlet to chamber 4. A further diffuser 22 has been added at the downstream, right-hand end of internal pipe 9. The six end caps 43 upstream of the catalyser modules have been removed. Each catalyser module is no longer divided into an upstream DENOX part and a downstream slip reducing part, but the entire catalyser modules has its channel walls covered by the same catalytic layer performing DENOX only. Three groups of matrix structures, 51, 52, and 53, have been inserted into each of the three chambers 4, 5, and 6.

Matrix structure 51 inside chamber 4 mainly serves two purposes: First, it ensures mixing of urea with gas as well as urea evaporation, due to the higher degree of local turbulence of gas flow created around elements of the matrix structure. Second, a sub-part 54 of matrix structure 51, arranged in front of the central zone of diffuser 21, ensuring that three will be no flow separation, as in explained in an akin arrangement shown previously. Matrix structure 52 has its interior surfaces in contact with through-flowing gas provided with a catalytic layer adapted for performing ‘slip’ elimination, that provides substitute for the downstream catalyser parts 42 of the prior art apparatus. Matrix structure 52 has been shown to be of a simple outer shape, to be of regular structure and not to extend to be left beyond outlets of the catalyser modules. A more refined matrix structure could have been made according and more matrix catalyser volume could have been accommodated to perform more slip reduction and to shape the flow pattern inside chamber 5 so as to enhance the performance of matrix structure 52. On the other hand, the simplicity of matrix structure 52 in itself is appealing, and it may sufficient and work well, depending on circumstances. The individual screens making up the matrix can be manufactured from a succession of sheets of nets that are cut out from bigger nets to their outer contours, and the seven holes needed for inserting the nets in-between pipe 9 and the six catalyser modules can be cut or stamped out of the nets.

Matrix structure 53 is more complicated. It can be divided into nine parts: Two parts 55 and 56 being arranged adjacent to each other, a third circular part, 57, being arranged in front of diffuser 22 and an total six circular parts 58 to 63. The diffuser slows down the inflow of gas to cavity 6, which in eases gas flow distribution inside the cavity, apart from recovering static pressure. To the right of the matrix structure there is a flat, empty space 64. Part matrix structure 57 helps prevent flow separation within diffuser 22.

Six similar matrix parts 58 to 63 provide extra flow resistance in front of the catalyser modules to promote an equal flow distribution among the modules, as well as suppressing tendencies to skewed flow across the inlets to the modules. These six matrix parts can be seen as direct replacements for the six caps 41 in the prior art apparatus. As can be seen, the extra flow resistances within part structures 57 to 63 has been attained by inserting extra, masked, circular sheets between bigger sheets. As can be seen from FIG. 28 a, matrix part 55 has more narrow masks than part matrix 56. This refinement will compensate for modules 32, 34, and 36 being arranged at a greater distance from inflow to chamber 6 than modules 31, 33, and 35, so that part gas flows inside chamber 6 will pass approximately the same amounts of matrix surface when flowing from the inlet into the individual modules.

The empty space 64 assists in easing flows to modules arranged at a relatively greater distance from inlet to chamber 6, as well as ensuring that, as viewed for each module, channels being situated further away from chamber inlet than other channels of the same module, will receive gas, i.e. counteracting skewed inflows to individual catalyser modules. In addition to modelling details of matrix structure 53 to optimise flow as has been described, all or some of matrix parts can be coated by a catalytic coating that will boost DENOX performance, either the same coating as that used inside the honeycomb catalyser modules, or another coating, adapted for the matrix, drawing upon its characteristics that differ from those of the honeycomb, or to perform a chemical reaction that differs from that being performed inside the honeycombs. Due to the bigger surface-to-volume ratio of fine-graded matrix elements, as compared to the walls of the honeycombs, a faster thermal response can be expected from the matrix structure. This can be taken advantage of to improve the SCR DENOX performance of the apparatus when subjected to varying operating conditions.

It has already been pointed out that inside cavity 5, to the left of gas outflows from modules, in the spacing between modules and division wall 7, more matrix structure could have been added. As an alternative to utilising this for adding more slip catalyser capacity, one could add more DENOX catalyser capacity by adding screens provided with a coating suitable for performing DENOX, upstream of slip reducing screens arranged as in the embodiment shown. Also, the structure of such added matrix could be adapted to compensate for any variation in the NOx concentration at outlets from individual channels from all six modules.

When deciding how to design details of matrix structure 52 one could proceed as follows: An apparatus will be designed with all the parts of the embodiment described, except for matrix 52. A range of velocity, NOx, and ammonia sensors is arranged at outlets from a number of channels, for instance three channels of each module being provided with anemometer velocity sensors, three other channels with NOx sensors, and three ammonia sensors at outlets of further channels of the same module, so that in total 54 sensors are inserted. They should be small enough and such arranged that their disturbance caused to the flow pattern inside chamber 5 will be minimal. If this cannot be achieved, the flow and chemical composition pattern can be mapped by transversing sensors; it will take more time to measure, but will disturb the flow less and will probably be cheaper to install.

From the patterns recorded, modifications improving the pattern can be adopted, perhaps by modifying the amount of urea added and making changes of matrix structures 51 and 52. Having performed a number of modifications, some minor deficiencies in flow and chemical composition pattern may remain. Having this knowledge will provide good input for designing matrix structure 52. As an overall consideration, one can subdivide matrix structure 52 into a DENOX part and a slip elimination part, the sizes of the two parts being tuned in conjunction with selecting the amount of urea that is being added. The optimisation procedure described may be refined to include, not only a single operation condition, but a test cycle, such as one of the standardised test cycles prescribed for testing the NOx and particulate emission performance of a particular engine provided with a certain exhaust emission treatment equipment.

Comparing pressure drops across the two apparatuses of FIGS. 27 a-c and 28 a-c at equal load conditions, the latter may have a smaller pressure drop, even though it contains more elements contributing to SCR DENOX function and slip elimination, resulting in lower NOx and slip concentrations downstream the apparatus. This can be attributed to two main factors: First, two pressure recovering diffusers 21 and 22 have been inserted. Second, although in particular circular matrix structure parts 57 to 63 have been selected specifically to locally provide added flow resistance, the pressure drop across the inflow and outflows from chamber 6, the outflows being inflows to catalyser modules, can be made smaller than the comparable pressure drop caused by caps 41, since in the matrix structure the pressure drop is more distributed, and the designer selecting details of the matrix structure and the design of the diffuser 22 has much more freedom to optimise performance.

In the embodiment shown in FIGS. 28 a-c, matrix structure 52, utilising the residual volume between standardised, cylindrical modules, has been connected in series gas flow with these modules. In further embodiments of the invention one may prefer a parallel connection. One could, for instance arrange such a matrix to perform the same chemical reaction as that performed by the modules. If the object is to reduce as much as possible a certain component of a gas, such as NOx or combustible matter in an oxidation process promoted by catalysis, one could select the design of the modules so that they will perform better than when all gas has to pass through the modules. This may or may not require any alteration of the channels of the modules, depending upon the specific chemical conversion taking place. In any case, a given total volume of catalytic modules should be capable of performing more reduction in terms of concentration of the noxious gas component leaving each module. Then one could add a matrix structure of such high density of catalytic surface per unit volume and with such a flow resistance that gas elements that have passed the matrix structure will attain at least the same low concentration of the noxious component as those gas elements that have passes the modules. The overall result will then be a greater reduction of the noxious component, thanks to a greater amount of catalytic surface being accommodated inside the given outer envelope.

Prior to the procedure of physical experimentation there may have been a phase of computer modelling performed to start out experiments on the basis of a better design than a design in which tailoring matrix structures has been made mainly on crude calculations and qualitative reasoning. Such simulation could start by pure computerised flow simulations, not including chemical reactions, according to one of the many commercial codes for such simulation that exist today on the market. A next step could be to include computer modelling of chemical reactions.

As the person skilled in the art will appreciate, optimisation of matrix structures operating inside a given apparatus, without or (as in the example above) in conjunction with honeycombs and/or foiled structures can be designed to be combined with experimentation in many ways, similar to many other previously made optimizations of systems performing chemical conversion be catalysis or otherwise, sometimes combined with silencing noise.

By adopting the method according to the claims when designing, constructing and/or using an apparatus and/or a silencer according to any of the claims an optimisation procedure will become possible to find clues for network configurations that one could hardly have figured out in more conventional ways. By drawing upon the vast capacity of modern computers one could afford, within reasonable means to test myriads of virtual configurations, even to include randomly defined matrix structures as a way of experimenting. Modelling can include one or a few calculated variables, such as for instance mean downstream NOx and slip concentrations. In this way it can be possible to distinguish more preferable matrix structure configurations from less preferable configurations.

FIGS. 29 a and b show a twenty-fifth embodiment of the invention, in which one or more metal foils have been adapted to constitute a matrix structure according to the invention. FIG. 29 a is a side view, and FIG. 29 b is a view from above. One can imagine that the structure extends in various directions, so that the figure shows part of a full embodiment. FIG. 29 a shows a succession of foiled layers on top of each other. These sheets could be rectangular, fitted into a rectangular cavity being fully covered by such sheets. Alternatively, the cavity may be circular, and the successive layers can be arranged by winding a long sheet of metal foil into a helical pattern that at its outer contour is adapted to the circular shape. There may also be an inner cylinder around which the helical structure is being wound.

The structure receives gas flow from the left. In FIG. 29 a all holes are arranged at the same horizontal position, so as to facilitate vertical flow. Viewed in FIG. 29 b, holes within the first three rows, 52, 53, and 54 are such arranged that spacings 60 are not aligned. Contrarily, from row 54 to row 56, all spacings 60 are aligned. Holes 51 have been created by stamping and bending small foil parts into members 61 and 62 that appear vertically in FIG. 29 a. Members 61 are longer than 62 and have ends touching an adjacent layer; by virtue of this they both act as spacers between individual layers and as a means of blocking flow across adjacent holes, as viewed in FIG. 29 b. Members 62 are shorter than members 61 and do not block flow, but provide some flow resistance.

It can be seen that due to the off-set arrangement of holes in rows 52-54, flow within a plane is forced to follow tortuous paths. This is combination with the previously mentioned fact of holes in the vertical plane being arranged in-line results in the structure providing a greater flow resistance in along the layers than in the perpendicular direction.

To arrange the opposite effect one could simple omit holes. The rear part of the assembly shown in FIGS. 29 a and b represents an intermediate situation. The relatively short members 62 enhance turbulence within the structure. In case of catalytic coating of the surfaces, such added turbulence will tend to promote chemical conversion.

A circular arrangement according to this embodiment could be adopted as a substitute for the previously described matrix structure, that is, to help distribute flow in a radial, outward direction. As with the previously described grid-like structure, such an effect can serve a triple purpose: firstly, to even out flow for the benefit of a downstream honeycomb catalyser, secondly, to allow for a more widening diffuser without major flow separation, and thirdly, to add catalyser capacity within a space that would otherwise be wasted from the point of view of cramming as much catalyser surface as possible into a limited space available.

A matrix structure according to FIGS. 29 a and b could be a separate unit from a downstream honeycomb, or it could be designed as an inlet, flow-pattern adapting structure of a honeycomb. 

1-101. (canceled)
 102. An apparatus arranged within a gas flow system comprising at least one matrix structure composed of a plurality of solid elements being arranged singularly or in groups, and said gas flow system comprising a plurality of voids in-between said solid elements, a majority of said singular solid elements or said groups of said solid elements being mechanically interconnected to a coherent mechanical structure including said voids, said matrix structure allowing gas to propagate within said matrix structure essentially along streamlines extending in any direction in space, directly or by circumventing said solid elements, thereby allowing for flow resistance to be significantly greater in one or more directions than in one or more other directions, said matrix structure being adapted to perform, solely by said matrix structure or in conjunction with other parts of said gas flow system, a change in the chemical composition of said gas having passed through the said system, due to one or more of the following processes taking place adjacent to the surfaces of said elements of said matrix structure: one or more chemical reactions promoted by catalytic coating of said elements, promoted mixing of different phases of components of said gas, or phase change by fluid drops evaporating or solid particles changing phase wherein the mechanical interconnection of said singular solid elements or said groups of said solid elements into the coherent mechanical structure including said voids is provided by at least one of the following mechanical structures: elongated mechanical members such as threads, strips or fibres having been arranged together by welding, brazing, soldering, fusing, weaving, spinning, intertwining or the like processes to create the voids, or one or more solid pieces of material having been work-processed such as having been processed by cutting, grinding, stamping, bending, stretching or etching out or the like work processes to create the voids, and wherein the said gas flow system additionally comprises at least one honeycomb structure in which gas flows predominantly inside longitudinal, essentially parallel flow channels, and wherein walls of the honeycomb structure are essentially impervious, said channels being open at both ends, and wherein at least one said matrix structure has been arranged upstream and/or downstream of said honeycomb structure and has been designed by adapting the flow resistance generally or in a specific an-isotropic way and/or by variation of said catalytic coating of said matrix that compensation is made for any non-uniform flow distribution of flow among said passages or channels of the said at least one honeycomb.
 103. An apparatus according to claim 102, wherein said matrix structure has been organised in a regular, partly regular or random way regarding inner details of the said structure.
 104. An apparatus according claim 102, wherein said streamlines are defined for gas elements that are bigger than a characteristic dimension of said elements and said voids, such as the thickness of a thread of the structure or the width of a mask between fibres of the structure, such that the flow pattern is viewed at a level of coarseness that is more coarse than details in said matrix structure, optionally wherein the degree of open structure of the matrix structure is defined by a dimensional resolution of the matrix structure that is significantly coarser than a characteristic dimension of said elements and said voids, such as the thickness of a thread of the structure or the width of a mask between fibres of the structure.
 105. An apparatus according to claim 102, wherein said gas when being allowed to flow through said matrix structure results in at least one of the following processes: contact between gas flowing through the said matrix structure to cause mixing of separate components or phases within the gas, phase change, one or more catalytic reactions promoted by catalytic material having been applied to a surface of the matrix structure, retaining of solid particles from the gas.
 106. An apparatus or a silencer according to claim 102, wherein the said matrix structure is constituted by at least one plane or curved grid or net with elongated elements such as threads or flat strips.
 107. An apparatus or a silencer according to claim 102, wherein the said matrix structure is constituted by at least one plane or curved perforated plate.
 108. An apparatus or a silencer according to claim 102, wherein the said matrix structure is composed of at least two groups of said matrix structures that are arranged adjacent or in-between each other, the first said group being of a significantly coarser structure than the second said group, the first said group providing significant mechanical strength of the said matrix, and the second said group adding an amount of mechanical surface per unit volume of the said matrix, thereby intensifying any process referred to in previous claims, one or more said groups carrying the same or differing types of catalytic coating, either to fit in much catalytic surface within a given space, or using differing catalytic coatings on elements of the differing groups in order to promote concurrent chemical reactions.
 109. An apparatus or silencer according to claim 106, wherein said honeycomb structure is at least partly catalytically coated and is having passages or channels that are open at both ends, and wherein said honeycomb structure comprises a plurality of elongated members extending inside the said passages or channels of the said honeycomb structure, at least part of said elongated members being catalytically coated.
 110. An apparatus or a silencer according to claim 102, wherein elongated members form part of or are attached to said matrix structure, and wherein the elongated members extend into said passages or channels of a said honeycomb structure, and wherein the elongated members are catalytically coated.
 111. An apparatus or a silencer according to claim 106, wherein said elongated members have been formed to a shape that deviates from a substantially straight shape, such as to comprise one or more curvatures within substantially a plane, or in sections of planes that may be tilted individually or may be aligned with each other, or to constitute a true 3-dimensional shape.
 112. An apparatus or a silencer according to claim 111, wherein the said elongated members or threads are shaped partly or wholly as a helical screw whose diameter can be changed elastically by applying torsion onto the said elongated members or threads.
 113. An apparatus or a silencer according to claim 106, wherein one or more of said honeycombs and said elongated members or treads have been catalytically coated either with essentially the same coating or by different coatings.
 114. An apparatus or a silencer according to claim 106, wherein said elongated members or said threads occupy at the most 50% of the cross-sectional area inside a said passage or channel, or at the most 25% of said cross-sectional area, or at the most 10% of the said cross-sectional area.
 115. An apparatus or a silencer according to claim 106, wherein individual elongated members or threads extends inside said passages or channels of a said honeycomb structure, and wherein said individual elongated members or said threads are of different lengths.
 116. An apparatus or a silencer according to claim 106, wherein individual elongated members or threads extends inside said passages or channels of a said honeycomb structure, and wherein said individual elongated members or said threads have different cross-sectional areas.
 117. An apparatus or a silencer according to claim 102, wherein said matrix structure is a catalyser being such arranged inside the said apparatus or silencer that an overall mean direction of flow direction can be singled out, the said catalyser comprising an inlet section to the said matrix structure, within which inlet section flow resistance in directions substantially perpendicular to the said mean direction of flow is significantly smaller than flow resistance in the direction of said mean flow direction.
 118. An apparatus or a silencer according to claim 102, wherein said at least one matrix structure has been arranged inside a chamber provided with at least one inlet passage and at least one outlet passage for gas to pass through the said chamber, the flow of gas, when passing said chamber, changing its direction by at least 45 degrees, the overall direction of flow passing through said at least one matrix structure also changing its overall flow direction by at least 45 degrees, and wherein said matrix structure is a catalyser.
 119. A silencer according to claim 106, wherein gas, when being led into a said chamber, is led via perforations of at least one inlet pipe extending forwards into said chamber, and where at least one said matrix structure surrounds said perforations in such a way that essentially all gas, when passing through said perforations of said inlet pipe, will also pass said matrix structure, and wherein said matrix structure is a catalyser.
 120. A silencer according to claim 106, wherein gas, when being led from a said chamber, passes through perforations of at least one outlet pipe extending backwards into the said chamber, and where at least one said matrix structure surrounds said perforations in such a way that essentially all gas, when passing through said perforations of said outlet pipe, will also pass said matrix structure, and wherein said matrix structure is a catalyser.
 121. A silencer according to claim 106, wherein gas, when being led into a said chamber, is led via an opening which is covered by said matrix structure being a catalyser, said opening being of such a shape and having such dimensions that essentially all gas, when entering said chamber, enters via said matrix structure by a gas flow pattern inside said matrix structure, said gas flow pattern generally diverging from said opening.
 122. A silencer according to claim 106, wherein gas, when being led from a chamber, is led via an opening which is covered by said matrix structure being a catalyser, said opening being of such a shape and having such dimensions that essentially all gas, when leaving said chamber, leaves via the said matrix structure by a gas flow pattern inside said matrix structure, said gas flow pattern generally converges towards said opening.
 123. An apparatus or a silencer according to claim 102, wherein said matrix structure is a catalyser and is placed in front of at least one opening for leading gas to a chamber in a predominant mean direction, said matrix structure being either circular of a diameter D or having another cross-sectional shape with a largest dimension D taken in a direction transverse to said predominant mean direction, a distance between said at least one opening and said matrix structure being less than two times said dimension D.
 124. An apparatus or a silencer according to claim 123, wherein said distance between said at least one opening and said matrix structure is less than the said dimension D.
 125. A silencer according to claim 106, wherein said matrix structure is arranged within a passage in such a way that gas flow, when passing through said passage, will also pass said matrix structure.
 126. A silencer according to claim 106, wherein said matrix structure is arranged inside a passage in such a way that a majority of cross-sections of said passage is only partly filled out by the said matrix structure.
 127. A silencer according to claim 106, wherein said matrix structure is arranged inside a passage in such a way that all cross-sections of said passage is only partly filled out by the said matrix structure.
 128. A silencer or an apparatus according to claim 102, wherein the flow pattern upstream of a said matrix structure generally rotates around an axis of said matrix structure.
 129. A silencer according to claim 106, wherein the flow pattern downstream of said matrix structure generally rotates around an axis of said matrix structure.
 130. A silencer according to claim 106, wherein at least one curved passage extends helically and essentially surrounds at least one said matrix structure.
 131. A silencer according to claim 106, wherein at least one curved passage extends helically and essentially surrounds at least one said honeycomb structure.
 132. A silencer according to claim 106, said silencer comprising at least a first and a second acoustic chamber being at least partly filled out by at least one said matrix structure, and wherein the first said chamber essentially surrounds the second said chamber.
 133. A silencer according to claim 106, said silencer comprising at least a first and a second acoustic chamber being at least partly filled out by at least one said honeycomb structure, and wherein the first said chamber essentially surrounds the second said chamber.
 134. A silencer according to claim 106, wherein said matrix structure is of an annular shape.
 135. A silencer according to claim 132, wherein said matrix structure is arranged inside the said first chamber, and wherein said first chamber is of an annular shape.
 136. A silencer according to claim 102, wherein two chambers are interconnected by an interconnecting member in which interconnecting member gas flow elements will pass from the one said chamber to the other said chamber along various flow lines, each said flow line being substantially longer than if said flow were to pass in an essentially rectilinear flow from the first said chamber to the second said chamber.
 137. A silencer according to claim 136, wherein said interconnecting member is capable of leading gas flow from said one chamber to said other chamber via at least one spirally or helically extending passage inside said interconnecting member.
 138. A silencer according to claim 106, wherein one or more of said matrix structures occupy an amount of space inside a chamber, said amount, apart from any space occupied by possible sound-absorptive material arranged inside said chamber, amounts to less than 10%, or between 10% and 30%, or between 30% and 50%, or between 50% and 70%, or between 70% and 90%, or more than 90% of the total space of a chamber of the said silencer.
 139. A silencer according to claim 106, wherein at least one said matrix structure occupies an amount of space inside a shell of the silencer, said amount, apart from any space occupied by possible sound-absorptive material and by possible thermal insulation material and by possible inner mechanical design members of said silencer, amounts to less than 10%, or between 10% and 30%, or between 30% and 50%, or between 50% and 70%, or between 70% and 90%, or more than 90% of the total space within said shell of said silencer.
 140. Use of a matrix structure for an apparatus or a silencer according to claim 102, wherein the matrix structure has an open structure in a degree that flow of gas can pass within said matrix structure essentially along streamlines extending in any direction in space, and wherein said matrix structure during use performs chemical conversion of gas, including optional retaining of suspended particles, flowing through the apparatus or the silencer.
 141. Use according to claim 140 of a matrix structure for an apparatus or a silencer, wherein the apparatus or the silencer also comprises at least one said honeycomb structure or foiled structure, and wherein said honeycomb structure or foiled structure also performs chemical conversion of gas, including optional retaining of suspended particles, flowing through the apparatus or the silencer.
 142. A combination of a honeycomb catalyser and a plurality of elongated members, said catalyser having essentially impervious walls surrounding channels that are open at both ends, said elongated members extending inside said channels and being catalytically coated.
 143. A method of optimising an apparatus or a silencer or one or more matrix structures according to claim 102 comprising one or more steps of computerised modelling a said apparatus or silencer or one or more matrix structures where one or more virtual matrix structures, representing a future, real matrix structure(s) of the same outer shape as the virtual structure(s), is/are incorporated, said virtual matrix structure(s) being defined by mathematical functions that are initially not necessarily being related to specific shapes or dimensions of the one or more matrices, but represent desired characteristics of the one or more virtual matrix structure(s), said virtual matrix structure(s) later to be replaced by physical matrix structure(s), either in computerised modelling based on known characteristics as they can be ascertained from a physical specification, or in a physical experiment, designing said physical matrix structure(s) such that their characteristics will crudely, as far as possible or rather closely match those of the virtual matrix structure(s).
 144. A method according to claim 143, wherein one or more of the following types of mathematical functions are defined: vector v*(x,y,z) of dimension m/s vector λ*(x, y, z) being dimensionless scalar c(x, y, z) of dimension: m²/m³=m⁻¹ one or more scalars m′(x,y,z), m″(x,y,z), . . . and one or more scalars r′(x, y, z), r″(x,y,z), . . . said vector v* representing the flow velocity at a given point x,y,z, said vector λ* representing an isotropic or an-isotropic length specific flow resistance in analogy with the dimensionless number λ=Δp/((L/d)0.5ρv²) representing the length specific flow resistance of pipe, flow propagating along the pipe, said scalar c representing the amount of reactive (sometimes catalytic) surface per unit volume, said one or more scalars m′, m″, . . . representing molar concentrations of one or more chemical constituents, and said one or more scalars r′, r″, . . . representing time rates of chemical reactions that will in general depend on concentrations, flow velocities, specific flow resistance, and possibly further space-varying variables, such as for instance temperature, such further variables to be defined in analogy with the variables that have been specified by way of example. 